Liquid crystal display apparatuses have been widely used as screens for word processors or computers. In recent years, a demand for such liquid crystal display apparatuses has been increasing rapidly also for TV screens. The liquid crystal display apparatuses are generally used in a TN (Twisted Nematic) mode; however, such liquid crystal apparatuses are liable to have problems when viewed obliquely, such as a reduction in contrast, a reversed gradation characteristic, etc. In response, recently, liquid crystal display apparatuses used in a VA (Vertical Alignment) mode have been viewed with interest. A liquid crystal cell in a liquid crystal display apparatus used in the VA mode is made up of a combination of a nematic liquid crystal having negative dielectric anisotropy and a vertical alignment film.
Recently, a liquid crystal cell having physical values and cell thickness which realizes a response speed in a sufficiently high level under the condition of a liquid crystal panel temperature of 25° C. has been developed. It should be noted here that the temperature of the liquid crystal panel generally becomes 10° C. higher than the ambient temperature by the heat generated from circuits surrounding liquid crystal cells, and the foregoing liquid crystal cells are applicable to apparatuses used in indoors without problem. However, when installing the apparatuses adopting the foregoing liquid crystal cells in the outside or applying liquid crystal cells to portable apparatuses, the response speed in a sufficiently high level may not be realized. Therefore, a liquid crystal cell which realizes a response speed in a sufficiently high level in any general use conditions has not yet been developed.
In response, for example, a driving method of liquid crystal cells in which a gradation transition is facilitated to realize a response speed is disclosed by Patent Document 1 (U.S. Pat. No. 2650479, published on Sep. 3, 1997). In this driving method, a voltage to be applied to a liquid crystal cell is corrected to facilitate a gradation transition. In this way, as compared to the case where the gradation transition is not facilitated, the luminance of the pixel in the level as desired can be attained in a shorter period of time.
However, even when adopting the above driving method to the foregoing conventional liquid crystal cell in a vertically aligned mode, if the response speed of the liquid crystal cell is not in the sufficient level, in such event that the luminance of the target level cannot be attained by the gradation transition from the last gradation to the current gradation, as it is assumed that the gradation transition made from the second last gradation to the last gradation is sufficient, a gradation transition may not be facilitated appropriately in the subsequent frame.
For example, as indicated by the solid line in FIG. 13, in the case where the gradation transition from the second last gradation to the current gradation is from a decay (in the direction of decreasing a luminance) to a rise (in the direction of increasing a luminance), if pixels are driven in the current frame FR (k) in the same manner as the case where the gradation transition is made in a sufficient level as indicated by the dashed line, despite of that the gradation transition made from the second last gradation to the last gradation is in fact insufficient, and the luminance at a start of the current frame FR (k) is not reduced to a sufficiently low level, a problem of generating an excessive brightness occurs resulting from an over facilitation of gradation transition.
On the other hand, as indicated by the solid line in FIG. 14, in the case where the gradation transition from the second last gradation to the current gradation is from rise to decay, if pixels are driven in the current frame FR (k) in the same manner as the case where the gradation transition is made in a sufficient level as shown by the dashed line, despite of that the gradation transition made from the second last gradation to the last gradation is in fact insufficient, and the luminance at a start of the current frame FR (k) is not raised to a sufficiently high level, a problem of generating a poor brightness occurs resulting from a over facilitation of gradation transition.
Incidentally, in the case where the gradation transition from the second last gradation to the current gradation is from a decay to a decay, if the gradation transition from the second last gradation to the last gradation is not made sufficiently, and the luminance at the start of the last frame FR(k-1) is not reduced to a sufficiently low level, the response speed of the liquid crystal in the current frame FR(k) is liable to be lowered. Similarly, in the case where the gradation transition from the second last gradation to the current gradation is from a rise to a rise, if the gradation transition from the second last gradation to the last gradation is not made sufficiently, and the luminance at the start of the last frame FR(k-1) is not raised to a sufficiently high level, the response speed of the liquid crystal in the current frame FR(k) is liable to be lowered.
As described, when adopting the foregoing driving method for the driving of the described conventional liquid crystal cells of the vertical alignment, a problem of reducing a display quality of the liquid crystal display apparatus is liable to occur if the gradation transition is facilitated in the same manner as the case where the gradation transition is made to a sufficient level, despite of that the response speed of a display element is not high enough, and the gradation transition is not in fact made sufficiently, a problem of deterioration of the display quality of the display apparatus is liable to occur resulting from an over facilitation of the gradation transition.
In applications of the liquid crystal display in environments of wider temperature range, because of the described characteristics of liquid crystals that their viscosities rise with a decrease in temperature, a problem of deteriorating the display quality is more liable to occur. In view of the foregoing, when it is expected to use under low temperature conditions, it is required to adopt a liquid crystal cell which offers an enough response speed to prevent deterioration of the display quality even under low temperature environments.
For example, under the condition of an ambient temperature of 0° C., since the panel temperature of the liquid crystal cell becomes around 5° C., it is required to adopt a liquid crystal cell which offers a high response speed enough to prevent the deterioration of the display quality under the condition of an ambient temperature of 5° C.
On the other hand, due to the limited selections for the applicable liquid crystal materials, cell thickness and application voltage, etc., a liquid crystal cell which offers a high response speed is more difficult to be manufactured as compared to the case of manufacturing a liquid crystal cell of low response speed. In view of this problem, provided that the deterioration of the display quality can be prevented, it is desirable to adopt liquid crystal cells of lower response speed in the aspect of manufacturing liquid crystal cells.